In the fields of, for example, regenerative medical engineering and tissue engineering, there are attempts to use various artificial materials as adjunctive materials for treatments or biological tissue replacements. Specific examples that actually have been put to practical use include an adhesion inhibitory material for preventing tissues from adhering to each other in surgical treatment as well as a scaffold for a cell culture, a biological tissue replacement, an artificial blood vessel, and an artificial trachea in regenerative medicine.
Such artificial materials used widely are, for example, polymer porous bodies. The polymer porous bodies can be produced by, for example, freeze-drying polymer solutions (Patent Documents 1 to 4). However, even among porous bodies formed of polymers in the same manner, they are required to have various properties and functions according to, for example, application sites in biological bodies and intended uses thereof.
The adhesion inhibitory material is placed on, for example, the surface of a tissue to be protected (for example, a damaged area) in the patient body. It is intended not to prevent the tissue to be protected from healing and is intended to prevent adhesion between the tissue to be protected and the surrounding tissue during the healing period. In order to fulfill this function satisfactorily, first of all, it is desirable that in the adhesion inhibitory material, the surface to be brought into contact with the tissue to be protected have excellent cell invasiveness. This improves adhesiveness between the adhesion inhibitory material and the tissue to be protected and thereby makes it possible to omit, for example, suturing. On the other hand, it is desirable that in the adhesion inhibitory material, the surface to be brought into contact with the surrounding tissue have, for example, less cell invasiveness. This can prevent cells of the surrounding tissue from invading the adhesion inhibitory material and adhering to the tissue to be protected. As described above, when the artificial porous body is used as an adhesion inhibitory material, for example, the surface to be brought into contact with a tissue to be protected and the opposite surface thereto (the surface to be brought into contact with a tissue that may adhere to the aforementioned tissue) are required to be completely different in property, function, and structure from each other.
Furthermore, there also are materials that are required to have excellent cell and substance invasiveness in both surfaces of the porous bodies depending on the intended uses thereof, which are different from the adhesion inhibitory material. Examples thereof include artificial blood vessels and artificial tracheae. A biological blood vessel has a three-layer membrane structure including an intima formed of endothelial cells, a tunica media formed mainly of smooth muscles, and an adventitia rich in connective tissues. Generally, the intima and adventitia serve in, for example, exchange of substances between blood and the external tissues. Furthermore, the adventitia also serves to maintain a blood vessel so as to prevent it from excessively dilating to rupture when, for example, the blood pressure increases. Moreover, the tunica media provides flexibility that copes with, for example, dilatation and constriction of the blood vessel. In this manner, since the respective parts provide specific functions, for example, the intima needs to have material permeability and blood compatibility, the tunica media flexibility, and the adventitia material permeability and physical strength, as their properties, respectively. Accordingly, artificial blood vessels also are required to have such properties as described above that are different from one another in the respective parts. However, since a porous body with good material permeability commonly has a large number of pores per area and also has a pore size that increases depending on the type of the substance to be permeated, it also has a problem of insufficient strength at the same time. Therefore, in order to avoid this problem, the external part of the porous body needs to have sufficiently high strength.
Furthermore, in order selectively to allow a specific cell to, for example, migrate, engraft, and proliferate according to the intended use, there is a demand for a material that allows specific cells to permeate and a material that does not allow cells to permeate but allows only substances such as nutrition and oxygen to permeate. In order to exhibit such selectivity, it is considered that a porous body with composite regions that are different in property and structure is desirable as compared to a porous body with uniform properties and structure.
As described above, the porous body is required to exhibit desired properties in desired positions according to the intended use thereof. Generally, however, available porous materials are homogeneous materials. Moreover, the process for controllably producing, for example, one whose opposed surfaces are different in property from each other or one whose inner part has different properties, according to need, has not been known. Accordingly, at present, it is necessary to prepare a plurality of porous materials that are different in property from each other beforehand and to laminate them in a desired order to form a porous body (Patent Documents 5 to 8). Furthermore, when a plurality of porous materials are laminated, it is desired that the respective layers adhere closely to one another and the resultant porous body be a unified product as a whole. However, with respect to polymer porous materials, the method of allowing a plurality of members to adhere to one another that has been reported is only a method of allowing them to adhere to one another with an adhesive such as a solvent (Patent Document 9). In the case of such a method, the process of producing a porous body requires an adhesion step additionally. Furthermore, safety of the adhesive and the residue thereof as well as adhesiveness between porous materials emerge as problems.    [Patent Document 1] JP 10 (1998)-234844 A    [Patent Document 2] JP 2001-49018 A    [Patent Document 3] JP 2002-541925 A    [Patent Document 4] JP 02 (1990)-2659935 A    [Patent Document 5] U.S. Pat. No. 5,607,474 A    [Patent Document 6] JP 2003-508128 A    [Patent Document 7] JP 2003-102755 A    [Patent Document 8] JP 02 (1990)-2659935 A    [Patent Document 9] U.S. Pat. No. 5,514,378 A